1. Field of the Invention
The present invention relates generally to a chemical mechanical polishing apparatus. More particularly, the present invention relates to the polishing head of a chemical mechanical polishing apparatus.
2. Description of the Related Art
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the surface of the wafer becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the wafer surface.
Chemical mechanical polishing (CMP) is a typical process used for this purpose. The CMP process is well-suited for use in connection with large-diameter wafers because the CMP process produces excellent uniformity in planarizing wide areas in addition to narrow ones.
The CMP process makes use of mechanical friction and a chemical agent for finely polishing a wafer surface. In the mechanical aspect of such polishing, a wafer is placed on a rotating polishing pad and is rotated while a predetermined load is applied thereto, whereby the wafer surface is polished by the friction created between the polishing pad and the wafer surface. In the chemical aspect of such polishing, the wafer surface is polished by a chemical polishing agent, referred to as slurry, supplied between the polishing pad and the wafer.
Typical CMP apparatus are disclosed in U.S. Pat. Nos. 5,423,716, 6,210,255, and 6,361,419. In these CMP apparatus, a wafer is held by a polishing head with the surface of the wafer to be polished (the process surface or polishing surface) facing a polishing pad. Then the wafer surface to be polished is placed against the polishing pad. At this time, the polishing head exerts a controllable pressure at the rear surface of the wafer.
More specifically, the polishing head includes a flexible membrane that provides a mounting surface to which the wafer is adhered, and a retaining ring to prevent the wafer adhered to the membrane from leaving the polishing head. The polishing head also includes a chamber and, and air inlets leading into the chamber. The membrane is expanded by feeding air into the chamber via the inlets. Thus, the load on the wafer is controlled by the amount of air fed into the chamber of the polishing head. Frequently, it is necessary to exert pressure on the wafer that varies from region to region across the wafer. To this end, a plurality of chambers may be formed in the polishing head and the membrane may include a fixing portion that extends upwards from the border between adjacent regions of the wafer and is fixed in a membrane supporter. The portions of the membrane corresponding to the various regions of the wafer are expanded when air is supplied into each of the chambers. However, a portion of the membrane corresponding to the border between the regions of the wafer, i.e., the portion of the membrane fixed to the membrane supporter, is not expanded. Accordingly, the lower surface of this portion of the membrane forms a concavity that prevents the CMP process from polishing the wafer with a high degree of uniformity.